Transcriptomic Analysis for Key Periods of Radicle Development in Contrasting Soybean Varieties HN75 and HN76

The root conformation of soybean is critical to achieve physiological activities such as nodulation and nitrogen fixation; however, the molecular determinants behind genotypic differences in its early development remain poorly described. In this study, we compared the characteristics of the soybean varieties HN75 and HN76 and examined developmental disparities in their root architectural characteristics and the transcriptomic profiles of radicles between them. The plant height and 100-grain weight of HN75, which had a longer growth cycle of 170 days, were slightly higher than those of HN76, which had a shorter growth cycle of 120 days. However, the numbers of pods and grains per plant were slightly lower. In terms of quality traits, HN75 had a higher oil content (23.40% versus 21.50%), whereas HN76 had a higher protein content (41.39% versus 35.71%). HN75 exhibited markedly superior root elongation (13.27 cm versus 10.15 cm), enhanced lateral root proliferation, and significantly greater nodule formation (19.53 versus 8.60 nodules per plant) relative to HN76 at 30 days post-germination, notwithstanding comparable nodule biomass. Chronobiological analysis (0–96 h post-germination) identified a pivotal developmental window of 48–72 h post-germination. Transcriptomic profiling of radicle tissues revealed 4792 differentially expressed genes (DEGs) in HN75 compared to 896 in HN76 during this critical interval, indicating substantially heightened transcriptional activity in HN75. Functional annotation enrichment demonstrated that HN75 DEGs were significantly enriched in phytohormone signalling cascades and isoprenoid biosynthetic pathways, whereas HN76 DEGs were predominantly associated with protein processing within the endoplasmic reticulum. We screened for eight genes (Glyma 10G071400, Glyma 13G057500, Glyma 08G016900, Glyma 09G028000, Glyma 18G265800, Glyma 03G032800, Glyma 02G064100, and Glyma 01G238600) that may play a role in the critical period of radicle development by performing network analyses and verified their dramatic changes in expression during this period by qRT-PCR. These results elucidate varietal-specific physiological and molecular mechanisms governing early radicle development in soybeans. These findings unravel mechanisms governing leguminous radicle development while establishing molecular blueprints for engineering cultivation protocols that would enhance soybean sustainability in edaphically constrained environments.